Stretch/cling films have wide application, including bundling packaged food and other goods. One application of particular interest is in the bundling of goods for shipping and storage, for example, the bundling of large rolls of carpet, fabric, or the like. A particularly important bundling application is the containment and unitizing of pallet loads.
Because the film can be under considerable tension in a stretched condition, it has a tendency to return to its original, unstretched state. This tension can cause the film to unravel from the wrapped pallet, thereby jeopardizing the integrity of the unitized load. A film having cling properties to prevent unraveling of the film from the pallet is therefore desirable.
To impart cling properties or improve the cling properties of a particular film, a number of techniques have been employed, such as the addition of tackifying additives or use of acrylates in the (co)polymer. Common tackifying additives include polybutenes, terpene resins, alkali metal and glycerol stearates and oleates and hydrogenated rosins and rosin esters.
The use of tackifiers has disadvantages. While tackification is known in the art to enhance cling in an olefin cling film, stretching the film normally reduces the cling properties. The tackifier may also present blending difficulties during film manufacture, adversely affect optical properties of the film and enhance surface migration of the additive. Such migration can damage the wrapped goods and cause the collapse and/or telescoping of the rolls. The use of compatible tackifiers has been described as a method to minimize the adverse effects such as degraded optical properties and migration in films.
Multilayer films offer the advantage of imparting several properties not easily obtained from a single layer. See U.S. Pat. Nos. 4,518,654 and 5,114,763 incorporated by reference. With a multilayer film, a stretch/cling wrap can have cling properties on one side and, for example, slip properties on the other.
Many of the multilayer films, however, suffer from the shortcomings of their individual layers. For instance, films containing a tackifying additive may be prepared and used in such a manner that the tackifier is "picked off" and onto the slip side of the film because the slip and cling layers of the film are in intimate contact on the film roll. Others do not possess desired slip properties, particularly when in a highly stretched state. Still others do not possess a desirable combination of stretch, tensile, tear-resistance, puncture-resistance, optical, and thermal stability properties.
Prior art cling films have typically contained either tackifier, as discussed above, and/or acrylate/acrylic monomer to impart the desired cling performance. It is desirable to have a film having cling properties without the adverse effects detailed above.
EP 0 287 272 and U.S. Pat. No. 5,248,547, assigned to Mobile Corporation, describe stretch wrap made from linear low density polyethylene (LLDPE) copolymers having a density of about 0.905 to about 0.940 gm/cm.sup.3 and a melt index (MI) of about 1 to about 10 dg/min, further containing from 4 to 10 weight percent of n-hexane extractables. These stretch wraps, in the absence of cling agents, reportedly, have inherent cling. The cling is disclosed as dependent on density and possibly also n-hexane extractables. EP 287 272 exemplifies Exxon LL3003.55 LLDPE (ethylene-hexene-1copolymer) containing approximately 6 weight percent n-hexane extractables possessing a cling force of 212 grams/inch (about 538 g/cm). The films as described by Mobil generally possess low molecular weight fractions which can impart undesirable characteristics such as difficulty in processing the copolymer into a film.